The diterpenoid phytohormone gibberellin (GA) controls diverse developmental processes in plants. DELLA proteins are nuclear transcriptional regulators, which function as master growth repressors by inhibiting all aspects of GA responses. Binding of GA to its nuclear receptor GID1 (for GA INSENSITIVE DWARF1) enhances the GID1-DELLA interaction, which in turn leads to the rapid proteolysis of DELLA through the ubiquitin-proteasome pathway, and allows transcriptional reprogramming of GA-responsive genes. This GA- GID1-DELLA signaling module appears to control plant growth and development by integrating internal signals from other hormone pathways [auxin, abscisic acid, jasmonic acid (JA) and ethylene], and external biotic (pathogen) and abiotic cues (light conditions, cold and salt stresses). Recent studies in Arabidopsis show that DELLA directly binds to and regulates key transcription factors/regulators in several signaling pathways (light, JA and developmental pathways). Importantly, these DELLA interactors belong to three different families of transcription factors. However, the current knowledge on how DELLA functions to regulate plant growth is only the tip of the iceberg. The goals of this proposal are to elucidate novel functions of selected newly identified DELLA interactors in GA-modulated plant development by genetic and biochemical approaches, and to determine how DELLA binds to multiple classes of interacting proteins at the molecular level by structure analyses. In addition to transcription factors, some of the new DELLA interactors are chromatin modification proteins, or O-GlcNAc transferase (OGT). O-GlcNAcylation of target proteins by OGT regulates a myriad of cellular pathways in animals, but its function in plants is poorly understood. This study will shed light on how OGT regulates signaling pathways in plants. The proposed study on the DELLA and its interacting regulatory proteins will contribute significantly to the mechanism by which DELLA modulates plant growth. This study will also provide new insights into the mechanisms that modulate transcriptional reprogramming and chromatin structure during growth and development, which are a common theme in controlling cell differentiation and tumorigenesis in humans.